In an effort to protect vehicle operators and occupants, vehicle manufacturers and their suppliers are increasingly designing and manufacturing vehicles with additional and improved safety features. One such safety feature that has been incorporated into a number of vehicle types is anti-lock braking, which can take a variety of forms.
Vehicle anti-lock brake systems are designed to maximize the ability of a vehicle operator to bring a vehicle to a controlled stop on any type of road surface. The system accomplishes this goal by preventing the vehicle brakes from prematurely halting vehicle wheel rotation, or "locking" the vehicle wheels, regardless of the road surface and the pressure applied to the brake pedal by the vehicle operator.
Typical vehicle anti-lock brake systems include vehicle wheel speed sensors for providing inputs to an anti-lock brake system control unit. The control unit controls anti-lock brake system control valves interposed between the brake master cylinder and the individual wheel brakes of a hydraulic brake circuit. Such control valves include isolation valves and dump valves. The control valves, in turn, regulate hydraulic brake fluid pressure in the individual wheel brakes to implement anti-lock braking.
In operation, one or more of the vehicle wheel speed sensors not only measures the vehicle wheel speed, but also provides input to the control unit for determining the vehicle speed. The control unit monitors the vehicle and vehicle wheel speeds for an indication of an anti-lock braking event. First, based upon the vehicle speed, the control unit typically determines a slip threshold. Using the vehicle velocity as a reference, slip threshold may be expressed as the difference between a selected velocity and the vehicle velocity.
Next, the control unit compares the vehicle wheel velocity to the vehicle velocity to determine a departure depth. Again using the vehicle velocity as a reference, departure depth may be expressed as the difference between the vehicle velocity and the wheel velocity. During normal vehicle braking, the wheel velocity closely matches the vehicle velocity. Thus, during normal vehicle braking, the difference between the vehicle velocity and the wheel velocity is nominal.
However, during an anti-lock braking event, the wheel velocity decreases significantly below, or "departs" from, the vehicle reference velocity. This is called "departure". In such a situation, as for example during hard braking on an ice covered road, the frictional force between the vehicle brake pads and the vehicle wheel exceeds that between the vehicle wheel and the road surface. Uncontrolled, such a frictional force differential causes the vehicle wheel to cease rotating, or to "lock."
In turn, locking causes the vehicle wheels to slip or "skid", rather than roll, over the road surface. Such vehicle wheel skidding dramatically reduces traction and the ability of the vehicle operator to bring the vehicle to a controlled stop.
To prevent such vehicle wheel lock and the accompanying problems, the control unit of an anti-lock brake system activates the anti-lock brake system isolation valve to regulate hydraulic brake fluid pressure in the individual wheel brakes during an anti-lock braking event. More specifically, the control unit compares the departure depth to the slip threshold and actuates the isolation valve when the departure depth exceeds the slip threshold in order to isolate the individual vehicle wheel brakes in the hydraulic brake circuit from the master cylinder, thereby halting any increase in brake fluid pressure in the vehicle wheel brakes and preventing incipient vehicle wheel lock.
More particularly, when, during vehicle braking, the departure depth exceeds the slip threshold, the isolation valve isolates brake fluid in the individual wheel brake from the increasing brake fluid pressure in the master cylinder in order to hold brake fluid pressure in the wheel brake constant. If the isolated brake fluid pressure in the wheel brake is still high enough to cause incipient wheel lock, the anti-lock brake system then bleeds, or dumps, brake fluid from the wheel brake through the dump valve to reduce brake fluid pressure therein.
Thereafter, the anti-lock brake system typically holds brake fluid pressure in the wheel brake constant until such time as the departure depth no longer exceeds the slip threshold, indicating that the vehicle wheel is again traveling at or near the velocity of the vehicle. At that time, the anti-lock brake system then increases, or builds, brake fluid thereto. Reapplication of brake fluid to the wheel brake may be at a steep or gradual rate, or some combination thereof, depending upon the circumstances or the control desired.
To maintain smooth braking and optimum vehicle control, some reapplication of brake fluid to the wheel brakes must be undertaken where the isolation of the brake fluid in the wheel brakes from that in the master cylinder has been prolonged, for example on the order of one hundred milliseconds or greater. Such reapplication must be undertaken in order to raise brake fluid pressure in the wheel brake to a level approximating that in the master cylinder before the isolation valve may deisolate the wheel brake from the master cylinder.
One problem associated with anti-lock brake systems as described above is their tendency toward false, or premature, activation. Premature activation can occur in a number of circumstances, such as where a road surface is partially ice covered or bumpy and individual vehicle wheels experience different coefficients of friction during braking. Typical anti-lock brake systems activate when the departure depth of any one of the vehicle wheels exceeds the slip threshold, despite the fact that the wheel would not have experienced excessive slip.
Moreover, typical anti-lock brake systems decrease the slip threshold after activation in order to increase anti-lock brake system sensitivity during an anti-lock braking event. Under normal conditions, this action improves the ability of the vehicle operator to slow or stop the vehicle in a controlled fashion.
At high speeds, typically over 50 mph, a vehicle may lose contact with a wet road which has a thin layer or film of water thereon. As a result, a wheel loses its capacity to transmit friction to the road surface and thus loses peripheral velocity. This is called hydroplaning or aquaplaning. During braking at such high speeds, the front wheels of the vehicle may experience deep and sustained slip and tend not to recover.
It is generally desirable to recognize this aquaplaning condition of a vehicle not only before braking but also after braking occurs. In such cases, once aquaplaning has been detected, the front wheels should be de-isolated and allowed to lock until the aquaplaning conditions are no longer present.
The U.S. patent to Leiber, U.S. Pat. No. 4,545,240, discloses a hydroplaning detection system which applies a low test braking pressure and measures wheel slip to determine the adhesion factor of the tires with respect to the road surface. If a given wheel slip occurs at a test pressure below that which would be expected, the driver can be alerted to the possibility of adverse road conditions. The test braking pressure can be applied manually or automatically.
The U.S. patent to Jonner, U.S. Pat. No. 4,059,312, discloses an ABS brake system which monitors the control signals for an inlet and outlet valve of each front wheel. When a valve for a first front wheel remains actuated for too long (e.g. 150 msec), the first wheel inlet and outlet valves will receive the second front wheel inlet and outlet control signals. If the inlet or outlet valves of both front wheels are actuated for too long (e.g. 150 msec), a first period begins (1.5 sec) to determine if hydroplaning is occurring. During this first time period, each front wheel inlet and outlet valve will receive its corresponding ABS control signal (i.e. left front wheel valves receive left front control signals). If valves for both front wheels remain actuated for the entire first period, ABS control of the front wheels will be switched off and a second time period (20 sec.) begins. The ABS control of the front wheels will be permanently switched off if the valves remain actuated throughout the second time period. If during the second time period, the braking operation ceases (i.e., the brake light goes out), ABS control is returned to the front wheels.
The U.S. patent to Jonner et al., U.S. Pat. No. 3,883,184, discloses an ABS brake system which monitors the control signals for an inlet and outlet valve of each front wheel in order to maintain faulty pressure control for the shortest possible time. If one of the valves is actuated longer than a predetermined time (150 msec for an outlet valve, 600 msec for an inlet valve), ABS control is disabled, unless the sensor signals present justify an extension of response time. If hydroplaning conditions exist (i.e. slip occurs at both front wheels), ABS control is not disabled and a second extended time period (20 seconds) is entered to verify that a fault does not exist. If the second time period expires and the slip signals persist, a fault is assumed and ABS control of all wheels is disabled.
Four control schemes for the '184 patent are possible:
1). If an outlet valve is actuated for said time and the front wheels do not slip, a defect is assumed and the ABS control is immediately disabled for the front wheels. The valves return to their unactuated position and the brakes are only pressurized by the master cylinder. PA1 2). If an outlet valve is actuated for said time and a front wheel does slip, but previously no deceleration signal or no slip signals were present from both wheels, it is not clear whether a defect can be assumed. In this case, ABS control is disabled and the brakes are pulsed (by a series of alternating pressure increases and reductions at the brake cylinders) for 20 seconds. If the slip signal remains at the end of this 20 second period, the ABS control is permanently disabled on the front wheels until the fault is corrected. PA1 3). If an outlet valve is actuated for said time and both front wheels slip, ABS control is continued for 20 seconds. If the slip signals continue after 20 seconds, the ABS control is disabled. PA1 4). If an outlet valve is actuated for said time and both front wheels slip or one front wheel slips and both front wheels decelerate, the pressure at both front wheels is reduced for 1 second and then pulsed for 20 seconds. If the slip remains at the end of the 20 seconds, the ABS control is disabled.
The U.S. patent to Yoshino, U.S. Pat. No. 5,020,863, discloses an ABS control system which monitors the brake pressure control signals for both front wheels for hydroplaning. A hydroplaning condition occurs when the control signals for reducing or alternatingly reducing and holding the brake pressure on both front wheels continuously exceed a predetermined duration. When hydroplaning is detected, the second control mode is entered wherein the rotational behavior for the front wheel with the highest wheel speed is used to derive the brake pressure control signals for both front wheels.